Well, you could learn how to
move dumpster trucks without causing yourself an injury -- but the key is time and conditioning.
Quick hit the nail on the head. Penile exercising is in fact healthy. The hardness and healthy benefits usually outweigh the size benefits (Imagine being able ejaculate without the erection subsiding -- some men experience newfound hardness like this).
However, like regular exercising, PE injuries can happen. I'll be honest with you. Most injuries take place when the exerciser wasn't using caution, or following his body's responses. The best advice I can give is to follow your PI's, and gradually move up in intensity. Here is the PI article that will help you avoid injuries and build a bigger, harder, and healthier penis:
http://exercisingthepenis.com/index.php?page=pis